Method of emission suppression in vacuum tubes, especially magnetron hats



June 24, 1958 E. c. oKREss ETAL 2,84 ,4 METHOD OF EMISSION SUPPRESSION m VACUUM TUBES, ESPECIALLY MAGNETRON HATS Filed Nov. 22, 1952 2 I Fig. l

Fig. 3.

WITNESSES: INVENTORS Ernest C. Okress 8| Slrfl [SW BYRolph J. Green w W/ v 5 W United States Patent METHOD OF EMISSION SUPPRESSION IN VACUUM TUBES, ESPECIALLY MAGNE- TRON HATS Ernest C. Okress, Montclair, and Ralph J. Green, Newark, N. 3., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application November 22, 1952, Serial No. 321,976

2 Claims. (Cl. 117-216) Our invention relates to graphite and metal-coatings and in particular relates to methods of applying coatings to surfaces of. metallic structural elements in electrical discharge devices to decrease their electron emission and simultaneously to increase thermal emissivity.

As is more fully describedin application Serial No. 319,032, now Patent No. 2,631,047, of Ernest E. Okress for Cathode End-Shields for Magnetrons, filed concurrently herewith, certain structural elements supported at the ends of the cylindrical hat-cathodes in present-day magnetrons tend to become thermionically or secondarily emissive of their own accord. or due to the contaminants thereon while the magnetrons are operating and to cause injurious arc-overs under the electric field-stresses existing in their vicinity. The said application describes and claims, among other things, the expedient of coating such structural elements with graphite to prevent such arcing. 7

While the coatings described in the Okress application have been found satisfactory in most conditions of operation, it has been found. that in tubes operating under conditions of extra high voltage-stress, any loosely deposited carbon is likely to be dislodged with consequent loss of effectiveness and may initiate arcing. While deposition of a more adherent carbon, namely graphite, by pyrolysis of hydrocarbons at more elevated temperatures is possible, such coatings have proven much less effective. than is required in. preventing electron emissions when using black sooty carbon.

One object of our invention is accordingly to provide a Way of forming on metallic surfaces a carbon coating which is permanently adherent while at the same time efiicient in preventing electron emission from causes re ferred to above.

Another object is to provide metallic surfaces with a carbon coating which is firmly adherent while at the same time having a high thermal emissivity.

Yet another object is to provide a method of coat-' Other objects of our invention will become apparent upon reading the following description taken in connection with the drawings in which:

Figure 1 is a schematic view indicating the porous structure of a metallic base-material used to fabricate electron tube electrodes and other structural elements in accordance with our invention;

Fig. 2 is a view partly in section of a magnetron 2 cathode provided with an end-shield made in accordance with our invention;

Fig. 3 is a diametral section of one form of the metallic base used informing the end-shield.

As has been briefly stated above, difliculty is found in. electron tube practice from the tendency of metal structures within the tube to emit electrons at undesired places which may produce a variety of undesirable effects such, for example, as arc-overs and insulation failures. Such electrons may, in the case of unheated structural elements, be due to secondary electron emission of the base metal itself or the active contaminants deposited thereon during normal operation. These contaminants normally come from the heated active cathode surface. The cause of this spurious emission is electron. and, ion bombardment and X-ray photo-induced. In the case of structures intentionally heated, or those unavoidably heated by proximity to neighboring elements, the emission may be thermionic as well. As is disclosed. and claimed in the Okress application previously mentioned, such electron-emission may be minimized by coating the metallic structures with a layer of graphite or certain other electron-suppressive and heat dissipative materials. In accordance with our present invention, superior coatings of this kind may be formed by making the underlying metal porous as indicated in Fig. '1, where the metal particles or crystals 1 are separated from each other by pores 2. The electron suppressive coating 3 may be graphite, zirconium, Zirconium oxide or other emission-suppressive and heat-dissipative material.

The porous underbody of metal may be formed in several different ways. A widely used metal for high voltage electron-tubeelectrode-supports and the like is tantalum, and: one method of forming the desired porous surface is to mold the structural element from powdered tantalum, as shown in Fig. 3, in a press and sinter the I resulting structure at a temperature suificiently below denum which ar'e frequently used in electron tube construction. V

Another method of forming metallic members of porous structure is moldingthem from unstable compounds of the metal, followed by subsequent thermal or other decomposition of the compound. Tantalum chloride and iodide are such decomposable compounds of tantalum, for example.

Still another method of forming metallic members with porous surfaces is molding them with small percentages of these unstable metal compounds, organic chemical compounds, or combinations of the two followed by subsequent thermal decomposition of the compounds. For example, camphor and naphthalene are such decomposable organic chemical compounds.

However, the method which we have so far found most satisfactory for most purposes in forming porous-surface metals is to mold them from a powdered mixture of the tantalum or other desired structural metal with a small percentage of a metal having a lower melting point or high vapor-pressure which does not alloy readily with it. While in many structures it will be desirable to thus mold the entire structure desired, the covering of a nonporous metal core with a coating of the aforesaid mixture by spraying, on a metal base as shown in Fig. 2, or compressing it to form, as shown in Fig. 3, are within the contemplation of our invention. T 0 give one instance, tantalum powder may be mixed with five to ten percent of powdered nickel to carry out the foregoing procedure.

After thus shaping the structural element, it may be ing point of the ancillary material, preferably in vacuum. This will leave the pores 2'of Fig. 1 between the metal particlesll iii-C ,I I

The electron-suppressive coating may be applied in one of several ways. For materials like zirconium, it may. be produced by condensation on the cooled metal surface of, zirconium vapor subliming from a body of that metal heated. in vacuum. A carbon coating may be formed by pyrolysis through heating the porous metal in an atmosphere of hydrocarbon vapor, e. g., heating the metal to 1500 to 1600 C. in an atmosphere of benzene. In many cases we have found that the structure thus coated may be outgassed, carboncoated and again outgassed, at a pressure of 10 mmfiof mercury in a single pumping operation.

Fig; 2 shows an example of a position on the cathode 12 of a magnetron. The cathode 12 has a thermionically-emissive surface 13 supported on a hollow sleeve or core 14 with provisions for heating surface 13. An anode structure 15 surrounds the cathode 12 leaving an annular end-chamber 16 from which it is desirable toprevent electrons emitted by the surface 13. The end-shield 11, maintained substantially at the potential of cathode-surface 13, cooperates with structural elementformed in accordance with our invention, an end-shield 11 in sistent with mechanical strength, are formed in the shell 18 around its line of attachment to sleeve 14.

' We claim as our invention:

1. The method of forming an electron-emission-suppressive surface on a refractory metal base member, said refractory metal being selected from the group consisting of tantalum, tungsten and molybdenum, said method including the steps of forming a non-porous surface layer of said refractory metal admixed with from 5 to 10% of a non-alloying material having a substantially higher vapor pressure than said refractory metal, said non-alloying material being selected from the group consisting of nickel, tantalum chloride, tantalum iodide, naphthalene and campho'r, heating said surface layer until a substantial portion of said non-alloying material is removed from said surface layer thereby forming a porous surface layer of approximately 5 to 10% porosity, and coating said porous surface layer with a continuous layer of low electron-emissive material selected from the groupconsisting' of graphite, zirconium and z irconium oxide.

2. The method of forming an electron-emission-sup' pressive surface on a tantalum base member, said method anode 15 to producean electric field which confines elec-.

trons, due to contamination of the surface 11 and its temperature, tothe annular space 24. Unfortunately it often becomes a source of thermionic and secondary electrons inhigh-voltage magnetrons, and so to facilitate arc-overs to the anode which are fatal to successful operation. Such undesired electron-emission from end-shield 11 may be minimized, as pointed out in the Okress application mentioned above, by covering the major portion of The end shield 11.1 is attached 1 including the steps of forming a non-porous surface layer of tantalum admixed with from 5 to 10% nickel, heating said surface layer until a substantial portion of nickel is removed from said surface layer thereby forming a porous surface layer of approximately 5 to 10% porosity, and coating said porous surface layer with a continuous layer of low electron-emissive material selected from the group consisting of graphite, zirconium and zirconium oxide.

References Cited in the file of this patent UNITED STATES PATENTS 927,935 Von Bolton July 13, 1900 'l,852,865 Upp 4-; Apr. 5, 1932 1,985,087 Glaser Dec 18, 1934 2,149,657 Armstrong Mar. 7, 1939 2,172,207 Kolligs Sept. 5, 1939 2,251,410 Koehring Aug. 5,1941 2,348,045 Wooten May 2, 1944 2,359,970 Clark Oct. 10, 1944 2,417,730 Becker Mar. 18, 1947 2,491,284 Sears Dec. 13, 1949 2,497,109 Williams Feb. 14,1950 2,683,671 Findlay July'l3, 1954 2,687,466 Mott Aug. 1954 

1. THE METHOD OF FORMING AN ELECTRON-EMISSION-SUPPRESSIVE SURFACE ON A REFRACTORY METAL BASED MEMBER, SAID REFRACTORY METAL BEING SELECTED FROM THE GROUP CONSISTING OF TANTALUM, TUNGSTEN AND MOLYBDENUM, SAID METHOD INCLUDING THE STEPS OF FORMING A NON-POROUS SURFACE LAYER OF SAID REFRACTORY METAL ADMIXED WITH FROM 5 TO 10% OF A NON-ALLOYING MATERIAL HAVING A SUBSTANTIALLY HIGHER VAPOR PRESSURE THAN SAID REFRACTORY METAL, SAID NON-ALLOYING MATERIAL BEING SELECTED FROM THE GROUP CONSISTING OF NICKEL, TANTALUM CHLORIDE, TANTALUM IODIDE, NAPHTHALENE AND CAMPHOR, HEATING SAID SURFACE LAYER UNTIL A SUBSTAN- 